Osteopontin (OPN), also known as bone sialoprotein I (BSP-1 or BNSP), early T-lymphocyte activation (ETA-1), secreted phosphoprotein 1 (SPP1), 2ar and Rickettsia resistance (Ric),[1] is a protein that in humans is encoded by the SPP1 gene (secreted phosphoprotein 1). The murine ortholog is Spp1. Osteopontin is a SIBLING (glycoprotein) that was first identified in 1986 in osteoblasts.

The prefix osteo- indicates that the protein is expressed in bone, although it is also expressed in other tissues. The suffix -pontin is derived from "pons," the Latin word for bridge, and signifies osteopontin's role as a linking protein. Osteopontin is an extracellular structural protein and therefore an organic component of bone. Synonyms for this protein include sialoprotein I and 44K BPP (bone phosphoprotein).

OPN is a highly negatively charged, extracellular matrix protein that lacks an extensive secondary structure.[2] It is composed of about 300 amino acids (297 in mouse; 314 in human) and is expressed as a 33-kDa nascent protein; there are also functionally important cleavage sites. OPN can go through posttranslational modifications, which increase its apparent molecular weight to about 44 kDa.[3] The OPN gene is composed of 7 exons, 6 of which containing coding sequence.[4][5] The first two exons contain the 5' untranslated region (5' UTR).[6] Exons 2, 3, 4, 5, 6, and 7 code for 17, 13, 27, 14, 108 and 134 amino acids, respectively.[6] All intron-exon boundaries are of the phase 0 type, thus alternative exon splicing maintains the reading frame of the OPN gene.

Figure 1. Proteolytic cleavage sites for full length osteopontin (OPN-FL). Thrombin exposes the cleaved epitope SVVYGLR (OPN-R), and then CPB removes the c-terminal arginine from OPN-R. The cleaved epitope has a non-RGD domain, which binds to integrin receptors (α4β1, α9β1, and α9β4). Next to the cleaved epitope, there is a RGD domain that interacts with other integrin receptors (αvβ1,3,5, and α5β1).

Full-length OPN (OPN-FL) can be modified by thrombin cleavage, which exposes a cryptic sequence, SVVYGLR on the cleaved form of the protein known as OPN-R (Fig. 1). This thrombin-cleaved OPN (OPN-R) exposes an epitope for integrin receptors of α4β1, α9β1, and α9β4.[7][8] These integrin receptors are present on a number of immune cells such as mast cells,[9] neutrophils,[10] and T cells. It is also expressed by monocytes and macrophages.[11] Upon binding these receptors, cells use several signal transduction pathways to elicit immune responses in these cells (See Section 3 for more detail). OPN-R can be further cleaved by Carboxypeptidase B (CPB) by removal of C-terminal arginine and become OPN-L (Fig. 2). The function of OPN-L is largely unknown.

It appears an intracellular variant of OPN (iOPN) is involved in a number of cellular processes including migration, fusion and motility.[12][13][14][15] Intracellular OPN is generated using an alternative translation start site on the same mRNA species used to generate the extracellular isoform.[16] This alternative translation start site is downstream of the N-terminal endoplasmic reticulum-targeting signal sequence, thus allowing cytoplasmic translation of OPN.

Various human cancers, including breast cancer, have been observed to express splice variants of OPN.[17][18] The cancer-specific splice variants are osteopontin-a, osteopontin-b, and osteopontin-c. Exon 5 is lacking from osteopontin-b, whereas osteopontin-c lacks exon 4.[17] Osteopontin-c has been suggested to facilitate the anchorage-independent phenotype of some human breast cancer cells due to its inability to associate with the extracellular matrix.[17]

Regulation of the osteopontin gene is incompletely understood. Different cell types may differ in their regulatory mechanisms of the OPN gene. OPN expression in bone predominantly occurs by osteoblasts and osteocyctes (bone-forming cells) as well as osteoclasts (bone-resorbing cells).[23] Runx2 (aka Cbfa1) and osterix (Osx) transcription factors are required for the expression of OPN [24] Runx2 and Osx bind promoters of osteoblast-specific genes such as Col1α1, Bsp, and Opn and upregulate transcription.[25]

Hypocalcemia and hypophosphatemia (instances that stimulate kidney proximal tubule cells to produce calcitriol (1α,25-dihydroxyvitamin D3)) lead to increases in OPN transcription, translation and secretion.[26] This is due to the presence of a high-specificity vitamin D response element (VDRE) in the OPN gene promoter.[27][28][29]

Extracellular inorganic phosphate (ePi) has also been identified as a modulator of OPN expression.[30]

Stimulation of OPN expression also occurs upon exposure of cells to pro-inflammatory cytokines,[31] classical mediators of acute inflammation (e.g. tumour necrosis factor α [TNFα], infterleukin-1β [IL-1β]), angiotensin II, transforming growth factor β (TGFβ) and parathyroid hormone (PTH),[32][33] although a detailed mechanistic understanding of these regulatory pathways are not yet known. Hyperglycemia and hypoxia are also known to increase OPN expression.[32][34][35]

OPN belongs to a family of secreted acidic proteins whose members have an abundance of negatively charged amino acids such as Asp and Glu.[36] OPN also has a large number of consensus sequence sites for post-translational phosphorylation of Ser residues to form phosphoserine, providing additional negative charge.[37] Contiguous stretches of high negative charge in OPN have been identified and named the polyAsp motif (poly-aspartic acid) and the ASARM motif (acidic serine- and asparate-rich motif), with the latter sequence having multiple phosphorylation sites.[38][39][40][41] This overall negative charge of OPN, along with its specific acidic motifs and the fact that OPN is an intrinsically disordered protein[42][43] allowing for open and flexible structures, permit OPN to bind strongly to calcium atoms available at crystal surfaces in various biominerals.[41][44][45] Such binding of OPN to various types of calcium-based biominerals ‒ such as calcium-phosphate mineral in bones and teeth,[46] calcium-carbonate mineral in inner ear otoconia[47] and avian eggshells,[48] and calcium-oxalate mineral in kidney stones[49][50][51] – acts as a mineralization inhibitor to regulate crystal growth.[52]

OPN is a substrate protein for a number of enzymes whose actions may modulate the mineralization-inhibiting function of OPN, a prominent one being PHEX (phosphate-regulating gene with homologies to endopeptidases on the X chromosome), which can extensively degrade OPN and whose inactivating gene mutations lead to altered processing of OPN and osteomalacia (soft hypomineralized bones) in X-linked hypophosphatemia (XLH).[53]

Along with its role in the regulation of normal mineralization within the extracellular matrices of bones and teeth,[54] OPN is also upregulated at sites of pathologic, ectopic calcification[55][56] – such as for example, in urolithiasis and vascular calcification ‒ presumably at least in part to inhibit debilitating mineralization in these soft tissues.

As discussed, OPN binds to several integrin receptors including α4β1, α9β1, and α9β4 expressed by leukocytes. These receptors have been well-established to function in cell adhesion, migration, and survival in these cells. Therefore, recent research efforts have focused on the role of OPN in mediating such responses.

Osteopontin (OPN) is expressed in a range of immune cells, including macrophages, neutrophils, dendritic cells, and T and B cells, with varying kinetics. OPN is reported to act as an immune modulator in a variety of manners.[2] Firstly, it has chemotactic properties, which promote cell recruitment to inflammatory sites. It also functions as an adhesion protein, involved in cell attachment and wound healing. In addition, OPN mediates cell activation and cytokine production, as well as promoting cell survival by regulating apoptosis.[2] The following examples are found.[2]

OPN plays an important role in neutrophil recruitment in alcoholic liver disease.[10][59] OPN is important for the migration of neutrophil in vitro.[60] In addition, OPN recruits inflammatory cells to arthritis joints in the collagen-induced arthritis model of rheumatoid arthritis.[61][62] A recent in vitro study in 2008 has found that OPN plays a role in mast cell migration.[63] Here OPN knock-out mast cells were cultured and they observed a decreased level of chemotaxis in these cells compared to wildtype mast cells. OPN was also found to act as a macrophage chemotactic factor.[64] In this study, researchers looked at the accumulation of macrophages in the brain of rhesus monkeys and found that OPN prevented macrophages from leaving the accumulation site, indicating an increased level of chemotaxis.

Activated T cells are promoted by IL-12 to differentiate towards the Th1 type, producing cytokines including IL-12 and IFNγ. OPN inhibits production of the Th2 cytokine IL-10, which leads to enhanced Th1 response. OPN influences cell-mediated immunity and has Th1 cytokine functions. It enhances B cell immunoglobulin production and proliferation.[2] Recent studies in 2008 suggest that OPN also induces mast cell degranulation.[63] The researchers here observed that IgE-mediated anaphylaxis was significantly reduced in OPN knock-out mice compared to wild-type mice. The role of OPN in activation of macrophages has also been implicated in a cancer study, when researchers discovered that OPN-producing tumors were able to induce macrophage activation compared to OPN-deficient tumors.[65]

Fig 2. Known immunologic functions of OPN. OPN binds to several integrin receptors including α4β1, α9β1, and α9β4 expressed by leukocytes and are known to induce cell adhesion, migration, and survival in immune cells including neutrophils, macrophages, T cells, mast cells, and osteoclasts.

OPN is an important anti-apoptotic factor in many circumstances. OPN blocks the activation-induced cell death of macrophages and T cells as well as fibroblasts and endothelial cells exposed to harmful stimuli.[66][67] OPN prevents non-programmed cell death in inflammatory colitis.[68]

The fact that OPN interacts with multiple cell surface receptors that are ubiquitously expressed makes it an active player in many physiological and pathological processes including wound healing, bone turnover, tumorigenesis, inflammation, ischemia, and immune responses1. Therefore, manipulation of plasma (or local) OPN levels may be useful in the treatment of autoimmune diseases, cancer metastasis, bone (and tooth) mineralization diseases, osteoporosis, and some forms of stress.[2]

OPN has been implicated in pathogenesis of rheumatoid arthritis. For instance, researchers found that OPN-R, the thrombin-cleaved form of OPN, was elevated in the rheumatoid arthritis joint. However, the role of OPN in rheumatoid arthritis is still unclear. One group found that OPN knock-out mice were protected against arthritis.[69] while others were not able to reproduce this observation.[70] OPN has been found to play a role in other autoimmune diseases including autoimmune hepatitis, allergic airway disease, and multiple sclerosis.[71]

Opn is up-regulated in inflammatory bowel disease (IBD). [74] Opn expression is highly up-regulated in intestinal immune and non-immune cells and in the plasma of patients with Crohn’s disease (CD) and Ulcerative colitis (UC), as well as in the colon and plasma of mice with experimental colitis. [74][75][76] Increased plasma Opn levels are related to the severity of CD inflammation, and certain Opn gene (Spp1) haplotypes are modifiers of CD susceptibility. Opn has also a proinflammatory role in TNBS- and dextran sulfate sodium (DSS)-induced colitis, which are mouse models for IBD. Opn was found highly expressed by a specific dendritic cell (DC) subset derived from murine mesenteric lymph nodes (MLNs)and is highly proinflammatory for colitis.[77] Dendritic cells are important for the development of intestinal inflammation in humans with IBD and in mice with experimental colitis. Opn expression by this inflammatory MLN DC subset is crucial for their pathogenic action during colitis.[77]

Osteopontin has recently been associated with allergic inflammation and asthma. Expression of Opn is significantly increased in lung epithelial and subepithelial cells of asthmatic patients in comparison to healthy subjects. [78] Opn expression is also upregulated in lungs of mice with allergic airway inflammation. [78] The secreted form of Opn (Opn-s) plays a proinflammatory role during allergen sensitization (OVA/Alum), as neutralization of Opn-s during that phase results in significantly milder allergic airway inflammation. [78] In contrast, neutralization of Opn-s during antigenic challenge exacerbates allergic airway disease. [78] These effects of Opn-s are mainly mediated by the regulation of Th2-suppressing plasmacytoid dendritic cells (DCs) during primary sensitization and Th2-promoting conventional DCs during secondary antigenic challenge.[78] OPN deficiency was also reported to protect against remodeling and bronchial hyperresponsiveness (BHR), again using a chronic allergen-challenge model of airway remodeling.[79] Furthermore, it was recently demonstrated that OPN expression is upregulated in human asthma, is associated with remodeling changes and its subepithelial expression correlates to disease severity.[80] OPN has also been reported to be increased in the sputum supernatant of smoking asthmatics,[81] as well as the BALF and bronchial tissue of smoking controls and asthmatics.[82]

Evidence is accumulating that suggests that osteopontin plays a number of roles in diseases of skeletal muscle, such as Duchenne muscular dystrophy. Osteopontin has been described as a component of the inflammatory environment of dystrophic and injured muscles,[22][83][84][85] and has also been shown to increase scarring of diaphragm muscles of aged dystrophic mice.[86] A recent study has identified osteopontin as a determinant of disease severity in patients with Duchenne muscular dystrophy.[87] This study found that a mutation in the osteopontin gene promoter, known to cause low levels of osteopontin expression, is associated with a decrease in age to loss of ambulation and muscle strength in patients with Duchenne muscular dystrophy.